Network Working Group P. Hoffman
Request for Comments: 2368 Internet Mail Consortium
Updates: 1738, 1808 L. Masinter
Category: Standards Track Xerox Corporation
J. Zawinski
Netscape Communications
July 1998
The mailto URL scheme
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1998). All Rights Reserved.
Abstract
This document defines the format of Uniform Resource Locators (URL)
for designating electronic mail addresses. It is one of a suite of
documents which replace RFC 1738, 'Uniform Resource Locators', and
RFC 1808, 'Relative Uniform Resource Locators'. The syntax of
'mailto' URLs from RFC 1738 is extended to allow creation of more RFC822 messages by allowing the URL to express additional header and
body fields.
1. Introduction
The mailto URL scheme is used to designate the Internet mailing
address of an individual or service. In its simplest form, a mailto
URL contains an Internet mail address.
For greater functionality, because interaction with some resources
may require message headers or message bodies to be specified as well
as the mail address, the mailto URL scheme is extended to allow
setting mail header fields and the message body.
2. Syntax of a mailto URL
Following the syntax conventions of RFC 1738 [RFC1738], a "mailto"
URL has the form:
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RFC 2368 The mailto URL scheme July 1998
mailtoURL = "mailto:" [ to ] [ headers ]
to = #mailbox
headers = "?" header *( "&" header )
header = hname "=" hvalue
hname = *urlc
hvalue = *urlc
"#mailbox" is as specified in RFC 822 [RFC822]. This means that it
consists of zero or more comma-separated mail addresses, possibly
including "phrase" and "comment" components. Note that all URL
reserved characters in "to" must be encoded: in particular,
parentheses, commas, and the percent sign ("%"), which commonly occur
in the "mailbox" syntax.
"hname" and "hvalue" are encodings of an RFC 822 header name and
value, respectively. As with "to", all URL reserved characters must
be encoded.
The special hname "body" indicates that the associated hvalue is the
body of the message. The "body" hname should contain the content for
the first text/plain body part of the message. The mailto URL is
primarily intended for generation of short text messages that are
actually the content of automatic processing (such as "subscribe"
messages for mailing lists), not general MIME bodies.
Within mailto URLs, the characters "?", "=", "&" are reserved.
Because the "&" (ampersand) character is reserved in HTML, any mailto
URL which contains an ampersand must be spelled differently in HTML
than in other contexts. A mailto URL which appears in an HTML
document must use "&amp;" instead of "&".
Also note that it is legal to specify both "to" and an "hname" whose
value is "to". That is,
mailto:addr1%2C%20addr2
is equivalent to
mailto:?to=addr1%2C%20addr2
is equivalent to
mailto:addr1?to=addr2
8-bit characters in mailto URLs are forbidden. MIME encoded words (as
defined in [RFC2047]) are permitted in header values, but not for any
part of a "body" hname.
Hoffman, et. al. Standards Track [Page 2]

RFC 2368 The mailto URL scheme July 19983. Semantics and operations
A mailto URL designates an "internet resource", which is the mailbox
specified in the address. When additional headers are supplied, the
resource designated is the same address, but with an additional
profile for accessing the resource. While there are Internet
resources that can only be accessed via electronic mail, the mailto
URL is not intended as a way of retrieving such objects
automatically.
In current practice, resolving URLs such as those in the "http"
scheme causes an immediate interaction between client software and a
host running an interactive server. The "mailto" URL has unusual
semantics because resolving such a URL does not cause an immediate
interaction. Instead, the client creates a message to the designated
address with the various header fields set as default. The user can
edit the message, send this message unedited, or choose not to send
the message. The operation of how any URL scheme is resolved is not
mandated by the URL specifications.
4. Unsafe headers
The user agent interpreting a mailto URL SHOULD choose not to create
a message if any of the headers are considered dangerous; it may also
choose to create a message with only a subset of the headers given in
the URL. Only the Subject, Keywords, and Body headers are believed
to be both safe and useful.
The creator of a mailto URL cannot expect the resolver of a URL to
understand more than the "subject" and "body" headers. Clients that
resolve mailto URLs into mail messages should be able to correctly
create RFC 822-compliant mail messages using the "subject" and "body"
headers.
5. EncodingRFC 1738 requires that many characters in URLs be encoded. This
affects the mailto scheme for some common characters that might
appear in addresses, headers or message contents. One such character
is space (" ", ASCII hex 20). Note the examples above that use "%20"
for space in the message body. Also note that line breaks in the
body of a message MUST be encoded with "%0D%0A".
People creating mailto URLs must be careful to encode any reserved
characters that are used in the URLs so that properly-written URL
interpreters can read them. Also, client software that reads URLs
must be careful to decode strings before creating the mail message so
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RFC 2368 The mailto URL scheme July 1998
that the mail messages appear in a form that the recipient will
understand. These strings should be decoded before showing the user
the message.
The mailto URL scheme is limited in that it does not provide for
substitution of variables. Thus, a message body that must include a
user's email address can not be encoded using the mailto URL. This
limitation also prevents mailto URLs that are signed with public keys
and other such variable information.
6. Examples
URLs for an ordinary individual mailing address:
<mailto:chris@example.com>
A URL for a mail response system that requires the name of the file
in the subject:
<mailto:infobot@example.com?subject=current-issue>
A mail response system that requires a "send" request in the body:
<mailto:infobot@example.com?body=send%20current-issue>
A similar URL could have two lines with different "send" requests (in
this case, "send current-issue" and, on the next line, "send index".)
<mailto:infobot@example.com?body=send%20current-
issue%0D%0Asend%20index>
An interesting use of your mailto URL is when browsing archives of
messages. Each browsed message might contain a mailto URL like:
<mailto:foobar@example.com?In-Reply-
To=%3c3469A91.D10AF4C@example.com>
A request to subscribe to a mailing list:
<mailto:majordomo@example.com?body=subscribe%20bamboo-l>
A URL for a single user which includes a CC of another user:
<mailto:joe@example.com?cc=bob@example.com&body=hello>
Another way of expressing the same thing:
<mailto:?to=joe@example.com&cc=bob@example.com&body=hello>
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RFC 2368 The mailto URL scheme July 1998
Note the use of the "&" reserved character, above. The following
example, by using "?" twice, is incorrect:
<mailto:joe@example.com?cc=bob@example.com?body=hello> ; WRONG!
According to RFC 822, the characters "?", "&", and even "%" may occur
in addr-specs. The fact that they are reserved characters in this URL
scheme is not a problem: those characters may appear in mailto URLs,
they just may not appear in unencoded form. The standard URL encoding
mechanisms ("%" followed by a two-digit hex number) must be used in
certain cases.
To indicate the address "gorby%kremvax@example.com" one would do:
<mailto:gorby%25kremvax@example.com>
To indicate the address "unlikely?address@example.com", and include
another header, one would do:
<mailto:unlikely%3Faddress@example.com?blat=foop>
As described above, the "&" (ampersand) character is reserved in HTML
and must be replacded with "&amp;". Thus, a complex URL that has
internal ampersands might look like:
Click
<a href="mailto:?to=joe@xyz.com&amp;cc=bob@xyz.com&amp;body=hello">
mailto:?to=joe@xyz.com&amp;cc=bob@xyz.com&amp;body=hello</a> to
send a greeting message to <i>Joe and Bob</i>.
7. Security Considerations
The mailto scheme can be used to send a message from one user to
another, and thus can introduce many security concerns. Mail messages
can be logged at the originating site, the recipient site, and
intermediary sites along the delivery path. If the messages are not
encoded, they can also be read at any of those sites.
A mailto URL gives a template for a message that can be sent by mail
client software. The contents of that template may be opaque or
difficult to read by the user at the time of specifying the URL.
Thus, a mail client should never send a message based on a mailto URL
without first showing the user the full message that will be sent
(including all headers that were specified by the mailto URL), fully
decoded, and asking the user for approval to send the message as
electronic mail. The mail client should also make it clear that the
user is about to send an electronic mail message, since the user may
not be aware that this is the result of a mailto URL.
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RFC 2368 The mailto URL scheme July 1998
A mail client should never send anything without complete disclosure
to the user of what is will be sent; it should disclose not only the
message destination, but also any headers. Unrecognized headers, or
headers with values inconsistent with those the mail client would
normally send should be especially suspect. MIME headers (MIME-
Version, Content-*) are most likely inappropriate, as are those
relating to routing (From, Bcc, Apparently-To, etc.)
Note that some headers are inherently unsafe to include in a message
generated from a URL. For example, headers such as "From:", "Bcc:",
and so on, should never be interpreted from a URL. In general, the
fewer headers interpreted from the URL, the less likely it is that a
sending agent will create an unsafe message.
Examples of problems with sending unapproved mail include:
* mail that breaks laws upon delivery, such as making illegal
threats;
* mail that identifies the sender as someone interested in breaking
laws;
* mail that identifies the sender to an unwanted third party;
* mail that causes a financial charge to be incurred on the sender;
* mail that causes an action on the recipient machine that causes
damage that might be attributed to the sender.
Programs that interpret mailto URLs should ensure that the SMTP
"From" address is set and correct.
8. IANA Considerations
This document changes the definition of the mailto: URI scheme; any
registry of URI schemes should refer to this document rather than its
predecessor, RFC 1738.
Hoffman, et. al. Standards Track [Page 6]

RFC 2368 The mailto URL scheme July 1998A. Change from RFC 1738RFC 1738 defined only a simple 'mailto' with no headers, just an
addr-spec (not a full mailbox.) However, required usage and
implementation has led to the development of an extended syntax that
included more header fields.
B. Acknowledgments
This document was derived from RFC 1738 and RFC 1808 [RFC1808]; the
acknowledgments from those specifications still applies.
The following people contributed to this memo or had and discussed
similar ideas for mailto.
Harald Alvestrand
Bryan Costales
Steve Dorner
Al Gilman
Mark Joseph
Laurence Lundblade
Keith Moore
Jacob Palme
Michael Patton
Hoffman, et. al. Standards Track [Page 8]

RFC 2368 The mailto URL scheme July 1998D. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Hoffman, et. al. Standards Track [Page 10]